The present invention relates to a direct current (DC) offset compensating circuit for use in an audio processing system. More particularly it relates to a DC offset compensating circuit for detecting a DC offset at an output terminal of a high-quality high-fidelity (Hi-Fi) amplifier and for compensating therefor.
Generally, in an audio system hi-fi amplifier, a coupling capacitor is not used since it causes a degradation in tone quality. While degradation in tone quality can be prevented in such a case where a coupling capacitor is not used, the resulting DC offset caused by asymmetry in positive and negative power swings of the output signal causes distortion in tone quality. Therefore, in order to improve tone quality it is desirable to compensate for the resulting DC offset.
FIG. 1 is a diagram of a conventional DC offset compensating circuit. A signal input through an input terminal P1 is applied to the gate of a transistor Q1, and this signal is applied, respectively, to the bases of transistors Q5 and Q6 via a differential amplifier which includes transistors Q1 to Q4. The signal applied to transistors Q5 and Q6 is differentially amplified and output through resistor R7. When the signal is differentially amplified by transistors Q5 and Q6, transistors Q9 and Q10 apply a reference level to each of their own bases by way of a Zener diode ZD1, thereby controlling a differentially amplified output signal in a uniform manner. The signal differentially amplified by transistors Q5 and Q6 is applied to the base of a transistor Q7, thereby driving transistor Q7. This signal is output to an output terminal OUT of the audio amplifier through a resistor R9. The offset of the signal output to output terminal OUT is the most dominant component of the signal responsible for impairing the tone quality of the hi-fi amplifier etc., and therefore, it is desirable to compensate for this offset.
Accordingly, in the conventional DC offset compensating circuit the signal output to output terminal OUT is fed back to a negative terminal (-) of an operational amplifier OP1 through a resistor R14. Operational amplifier OP1 detects the amount by which the output signal deviates from a "O" level. More specifically, a positive terminal of operational amplifier OP1, is connected to a resistor R15 and a capacitor C2, which thereby defines a time constant value for a reference level.
Operational amplifier OP1 detects the case in which the offset of the output signal deviates slightly in a negative(-) direction from a "O" voltage. Upon detecting such a deviation, operational amplifier OP1 outputs an inverted form of the detected offset. The signal output from operational amplifier OP1 is applied to the base of transistor Q6 and to a drain of transistor Q3, which then causes the bias to shift to a positive(+) value. That is, in transistors Q5 and Q6 the DC balance moves to the positive(+) side of the "0" voltage level. Thereby, the negative(-) DC offset value applied to the negative terminal (-) of operational amplifier OP1 moves towards the "O" level so as to be compensated, thereby maintaining an overall DC balance at the "O" level.
As described above in regards to the conventional offset compensation circuit, since an offset remains in the detection feedback loop of operational amplifier OP1, the circuit is limited in achieving a perfect DC offset compensation, even though this offset is compensated to be near to the "0" level. Therefore, the audio system hi-fi amplifier employing the conventional offset compensation circuit suffers from the problem of inferior tone quality.